Manuel Vilas-Varela

Skeletal editing enables precise structural modifications of molecules at late stages of a synthetic sequence. We recently demonstrated skeletal editing on a single-molecule scale. Voltage pulses applied by the tip of a scanning probe microscope to an oxygen-containing seven-membered heterocycle led to both oxygen deletion and ring-contraction rearrangement reactions. An open question is whether selective skeletal editing of a heterocyclic core can be achieved by the choice of heteroatom. Here we show that tip-induced reactions of an analogous sulfur-containing seven-membered ring mainly result in sulfur deletion. Our results demonstrate that the combination of tip-induced chemistry and heteroatom selection in the molecular design is a powerful strategy for single-molecule skeletal editing, potentially enabling diverse structural transformations of heterocyclic frameworks.

Electrically conductive two‐dimensional covalent organic frameworks (2D COFs) have emerged as a versatile class of crystalline porous polymers with promising applications in electronics and energy storage. However, high electrical conductivity generally relies on post‐synthetic doping to generate charge carriers, which can compromise crystallinity, porosity, and structural homogeneity. Persistent neutral radical conductors provide an attractive alternative, as their unpaired electrons can generate free charge carriers without the need for counterions. Nevertheless, the incorporation of highly spin‐delocalized π‐radicals into COFs remains largely unexplored. Herein, we report the design and synthesis of imine‐linked 2D COFs incorporating spin‐delocalized trioxotriangulene (TOT) neutral radicals through complementary synthetic approaches. Direct use of a TOT radical derivative bearing amino groups affords a highly crystalline framework (TOT‐COF‐H) that exhibits semiconducting behavior ( σ _RT = 1.2 × 10 ⁻⁴ S cm ⁻¹ ), a reduced band gap ( E _g = 1.09 eV), and a low activation energy (0.24 eV). This work demonstrates a viable strategy for integrating spin‐delocalized neutral radical building blocks into COFs, enabling the development of intrinsically conductive, porous, and crystalline organic frameworks without the need for extrinsic doping.

Electrically conductive two‐dimensional covalent organic frameworks (2D COFs) have emerged as a versatile class of crystalline porous polymers with promising applications in electronics and energy storage. However, high electrical conductivity generally relies on post‐synthetic doping to generate charge carriers, which can compromise crystallinity, porosity, and structural homogeneity. Persistent neutral radical conductors provide an attractive alternative, as their unpaired electrons can generate free charge carriers without the need for counterions. Nevertheless, the incorporation of highly spin‐delocalized π‐radicals into COFs remains largely unexplored. Herein, we report the design and synthesis of imine‐linked 2D COFs incorporating spin‐delocalized trioxotriangulene (TOT) neutral radicals through complementary synthetic approaches. Direct use of a TOT radical derivative bearing amino groups affords a highly crystalline framework (TOT‐COF‐H) that exhibits semiconducting behavior ( σ _RT = 1.2 × 10 ⁻⁴ S cm ⁻¹ ), a reduced band gap ( E _g = 1.09 eV), and a low activation energy (0.24 eV). This work demonstrates a viable strategy for integrating spin‐delocalized neutral radical building blocks into COFs, enabling the development of intrinsically conductive, porous, and crystalline organic frameworks without the need for extrinsic doping.

Particle-exchange heat engines operate without moving parts or time-dependent driving, relying solely on static energy-selective transport. Here, we realize a particle-exchange quantum heat engine based on a single diradical molecule, which is only a few nanometers in size. We experimentally investigate its operation at low temperatures and demonstrate that both the power output and efficiency are significantly enhanced by Kondo correlations, reaching up to 53% of the Curzon-Ahlborn limit. These results establish molecular-scale particle-exchange engines as promising candidates for low-temperature applications where extreme miniaturization and energy efficiency are paramount.

Concealed non-Kekulé polybenzenoid hydrocarbons have no sublattice imbalance yet cannot be assigned a classical Kekulé structure, leading to an open-shell ground state with potential applications in organic spintronics. They constitute an exceedingly small fraction of the total number of polybenzenoid hydrocarbons that can be constructed for a given number of benzenoid rings, and their synthesis remains challenging. The archetype of such a system is the Clar's goblet (C₃₈H₁₈), a diradical proposed by Erich Clar in 1972 and recently synthesized on a Au(111) surface. Here, we report the synthesis of a π-extended Clar's goblet (C₇₆H₂₆), a tetraradical concealed non-Kekulé polybenzenoid hydrocarbon, by a combined in-solution and on-surface synthetic approach. By low-temperature scanning tunneling microscopy and atomic force microscopy, we characterized individual molecules adsorbed on a Cu(111) surface. We provide insights into the electronic properties of this elusive molecule, including the many-body nature of its ground and excited states, by mean-field and multiconfigurational quantum chemistry calculations.

Tailor‐made graphene nanostructures can exhibit symmetry‐protected topological boundary states that host localized spin‐1/2 magnetic moments at half filling. However, one frequently observes charge transfer on coinage metal substrates, which results in closed‐shell configurations. Using low‐temperature scanning tunneling spectroscopy, it is demonstrated here that pristine topologically nontrivial chiral graphene nanoribbons synthesized directly on the ferromagnet GdAu₂ can either maintain a charge‐neutral diradical state, or convert to a singly anionic doublet. As an underlying mechanism, both a work function and an exchange field modulated by the moiré‐induced superstructure are identified, as corroborated by Kelvin probe force microscopy and spin‐flip spectroscopy. The joint electrostatic and magnetic interactions allow reversibly switching between the three spin multiplicities by atomic manipulation. An effective Hubbard dimer model is introduced that unifies the effects of local electrostatic gating, electron–electron correlation, hybridization and an exchange field to outline the phase diagram of accessible spin states. These results establish a platform for the local control of π‐radicals adsorbed on metallic substrates.

Triangulenes are prototypical examples of open-shell nanographenes. Their magnetic properties, arising from the presence of unpaired π electrons, can be extensively tuned by modifying their size and shape or by introducing heteroatoms. Different triangulene derivatives have been designed and synthesized in recent years thanks to the development of on-surface synthesis strategies. Triangulene-based nanostructures with polyradical character, hosting several interacting spin units, can be challenging to fabricate but are particularly interesting for potential applications in carbon-based spintronics. Here, we combine pristine and N-doped triangulenes into a more complex nanographene, TTAT, predicted to possess three unpaired π electrons delocalized along the zigzag periphery. We generate the molecule on a Au(111) surface and detect direct fingerprints of multiradical coupling and high-spin state using scanning tunneling microscopy and spectroscopy. With the support of theoretical calculations, we show that its three radical units are localized at distinct parts of the molecule and couple via symmetric ferromagnetic interactions, which result in a S = 3/2 ground state, thus demonstrating the realization of a molecular ferromagnetic Heisenberg spin trimer.

We produce atomically precise cross-conjugated one-dimensional polymeric chains designed in their heteroatom content. The pyridine location and number in the chains effectively tune both the gap size and the frontier orbital energy positions.

Engineering sublattice imbalance is an intuitive way to induce high-spin ground states in bipartite polycyclic conjugated hydrocarbons (PCHs). Such molecules can be employed as building blocks of quantum spin chains, which are outstanding platforms to study fundamental models in quantum magnetism. This is exemplified by recent reports on the bottom-up synthesis of antiferromagnetic spin chains that provided insights into paradigmatic quantum phenomena such as fractionalization. In contrast to antiferromagnetism, demonstration of ferromagnetic coupling between PCHs has been scarce. Previous attempts in this direction were limited by the formation of nonbenzenoid rings leading to spin quenching or the use of spacer motifs that weaken the magnitude of ferromagnetic exchange. Here, we demonstrate the on-surface synthesis of short ferromagnetic spin chains based on dibenzotriangulene, a triplet PCH. Our synthetic strategy centers on the concept of achieving a direct (without spacer motifs) majority-minority sublattice coupling between adjacent molecules. This leads to a global sublattice imbalance in spin chains scaling with the chain length and therefore a ferromagnetic ground state with a strong intermolecular ferromagnetic exchange. Through scanning probe measurements and quantum chemical calculations, we analyze the electronic and magnetic properties of ferromagnetic dimers and trimers of dibenzotriangulene and confirm their quintet and septet ground states, respectively, with an intermolecular ferromagnetic exchange of 7 meV. Furthermore, we elucidate the role of sublattice coupling on magnetism through complementary experiments on antiferromagnetic dibenzotriangulene dimers with majority-majority and minority-minority sublattice couplings. We expect our study to provide impetus for the design of organic ferromagnetic materials.

Open-shell polycyclic aromatic hydrocarbons (PAHs) represent promising building blocks for carbon-based functional magnetic materials. Their magnetic properties stem from the presence of unpaired electrons localized in radical states of π character. Consequently, these materials are inclined to exhibit spin delocalization, form extended collective states, and respond to the flexibility of the molecular backbones. However, they are also highly reactive, requiring structural strategies to protect the radical states from reacting with the environment. Here, we demonstrate that the open-shell ground state of the diradical 2-OS survives on a Au(111) substrate as a global singlet formed by two unpaired electrons with antiparallel spins coupled through a conformational-dependent interaction. The 2-OS molecule is a "protected" derivative of the Chichibabin's diradical, featuring a nonplanar geometry that destabilizes the closed-shell quinoidal structure. Using scanning tunneling microscopy (STM), we localized the two interacting spins at the molecular edges, and detected an excited triplet state a few millielectronvolts above the singlet ground state. Mean-field Hubbard simulations reveal that the exchange coupling between the two spins strongly depends on the torsional angles between the different molecular moieties, suggesting the possibility of influencing the molecule's magnetic state through structural changes. This was demonstrated here using the STM tip to manipulate the molecular conformation, while simultaneously detecting changes in the spin excitation spectrum. Our work suggests the potential of these PAHs as all-carbon spin-crossover materials.